Acetylcholine is the neurotransmitter produced by neurons referred to as cholinergic neurons. In the peripheral nervous system acetylcholine plays a role in skeletal muscle movement, as well as in the regulation of smooth muscle and cardiac muscle. In the central nervous system acetylcholine is believed to be involved in learning, memory, and mood.
Acetylcholine is synthesized from choline and acetyl coenzyme A through the action of the enzyme choline acetyltransferase and becomes packaged into membrane-bound vesicles. After the arrival of a nerve signal at the termination of an axon, the vesicles fuse with the cell membrane, causing the release of acetylcholine into the synaptic cleft. For the nerve signal to continue, acetylcholine must diffuse to another nearby neuron or muscle cell, where it will bind and activate a receptor protein.
There are two main types of cholinergic receptors, nicotinic and muscarinic. Nicotinic receptors are located at synapses between two neurons and at synapses between neurons and skeletal muscle cells. Upon activation a nicotinic receptor acts as a channel for the movement of ions into and out of the neuron, directly resulting in depolarization of the neuron. Muscarinic receptors, located at the synapses of nerves with smooth or cardiac muscle, trigger a chain of chemical events referred to as signal transduction.
For a cholinergic neuron to receive another impulse, acetylcholine must be released from the receptor to which it has bound. This will only happen if the concentration of acetylcholine in the synaptic cleft is very low. Low synaptic concentrations of acetylcholine can be maintained via a hydrolysis reaction catalyzed by the enzyme acetylcholinesterase. This enzyme hydrolyzes acetylcholine into acetic acid and choline. If acetylcholinesterase activity is inhibited, the synaptic concentration of acetylcholine will remain higher than normal. If this inhibition is irreversible, as in the case of exposure to many nerve gases and some pesticides, sweating, bronchial constriction, convulsions, paralysis, and possibly death can occur. Although irreversible inhibition is dangerous, beneficial effects may be derived from transient (reversible) inhibition. Drugs that inhibit acetylcholinesterase in a reversible manner have been shown to improve memory in some people with Alzheimer's disease.
Acetylcholine helps with stimulating short-term memory. According to Williams College, as you age, the acetylcholine concentration in your body decreases, resulting in the development of occasional forgetfulness and lapses in short-term memory. This decline in acetylcholine causes Alzheimer's disease, which is seen mainly in the elderly population, where memory loss occurs. Williams College reports that with Alzheimer's disease, acetylcholine levels decrease up to 90 percent of normal levels. Furthermore, the nerve cells that respond to acetylcholine stimulation progressively die, resulting in the disruption in the processing of thoughts and behavioral deterioration of Alzheimer's patients.
The use of chemical forms of acetylcholine, such as acetylcholine chloride intraocular solution, may serve as an important medication to cause pupil constriction during eye surgical procedures. Drug Information Online reports this benefit is essential for patients undergoing eye surgery. Your pupil lies centrally within the eye, and the iris, which provides your eye color, surrounds it. Acetylcholine causes the muscles of the iris to contract, resulting in decreased diameter of the pupil, also known as pupil constriction. This important benefit allows the pupils to regain activity after cataract eye surgery.
Digestion and absorption of nutrients in your body occurs in the gastrointestinal or digestive tract. The hollow organs of the gastrointestinal tract, such as the stomach and the intestines, contain a muscular layer, which allows for the efficient coordinated, wave-like contraction of the organs, also known as peristalsis. This process propels food through your digestive tract. The National Digestive Diseases Information Clearinghouse states that acetylcholine increases the contractions seen in the muscular layer, thus improving peristalsis and pushing the food efficiently through the digestive tract.
The vagus nerve, which regulates the rate of your heartbeat, releases acetylcholine. The release of acetylcholine results in the slowing of your heartbeat from the normal rate of 100 beats per minute to 60 to 80 beats per minute. The importance of this function is that it prevents your heart rate from getting too high.
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